Herein, related art is described for expository purposes. Related art labeled “prior art”, if any, is admitted prior art; related art not labeled “prior art” is not admitted prior art.
Electrical power to an electrical load, in particular Alternating Current (AC) voltage, can be controlled by adjusting the time-averaged (e.g., RMS) voltage reaching the load. This is particularly applicable to incandescent lamp loads as modulating the RMS current supplied to lamps is not visible to the human eye due to the thermal mass of the lamp's filament. The RMS current can be modulated by controlling the peak-to-peak voltage of the AC voltage, e.g., varying a resistance inserted in series with the load, over which the supplied load voltage is dropped. However, the heat dissipated during the voltage drop can be excessive, nearly matching that of the load itself in the worst case.
An alternate method of controlling the RMS voltage supplied to a load is through the application of phase control to the AC voltage waveform. Phase control involves decoupling the load from the AC waveform for a portion of each AC half cycle so that the average voltage can be adjusted by adjusting the duty cycle during which the load is coupled to the AC waveform. In the case of phase control, relatively little heat is generated when the load is fully coupled or fully decoupled. However, many loads, such as incandescent lamps, are very sensitive to sharp transitions from a fully coupled to a fully decoupled state, as these rapid transitions tend to mechanically excite the lamp's filament and produce audible noise or “sing”. Additionally, such sharp voltage transitions will tend to radiate electromagnetic waves causing interference with other electronic equipment in the vicinity. To minimize these effects, nearly all prior-art load controllers incorporate some form of transition controls. However, controlling such transitions requires a controlled impedance be placed between the load and the AC power source, and such a load impedance device will generate significant thermal load while controlling the transition. Thus, phase control of loads, and in particular lighting loads, involves design tradeoffs between heat and electrical and audible noise.
Because most electrical load controllers are obliged to deliver a requested RMS output voltage in response to a control input, and because varying the transition time in response to thermal conditions changes the relative contribution made by the transition time of the waveform to the full RMS output of the controller, the preferred embodiment for this type of load controller will incorporate an ability to sense the output voltage and adjust the phase of the switching means to regulate the effective output. (This would be referred to as closed-loop control of the output voltage.) While prior-art approaches provide for voltage regulation through the adjustment of the phase, based upon an analysis of the AC line input waveform, such an approach does not admit to each controlled channel using a different transition time in response to its own thermal environment. Hence a channel-by-channel RMS voltage closed loop control is preferred for this invention.